Set your joists at 300mm centres. That is the spacing that keeps a composite deck feeling solid underfoot with no bounce or sag, and it is the maximum we allow on our Elite range under the 15-year guarantee. You can go tighter to around 250mm for a diagonal deck or one that takes heavy use, but do not stretch past 300mm. The 400mm and 600mm figures people find online are for structural floor joists, not for the boards on top, and using them under composite decking is the quickest way to end up with a springy, moving deck.

Joist spacing, or joist centres, is one of those numbers that gets muddled all the time, because the same word covers two different jobs. This is about how far apart the joists sit under your deck boards, measured centre to centre. Get it right and the deck feels like a solid floor. Get it wrong and you feel every step. Here is how we do it.

What joist spacing should you use for composite decking?

For our boards, 300mm centres is the number to work to. It suits both ranges and gives the boards enough support that they will not flex between the joists. On our capped Elite range it is a firm maximum, written into the warranty, because those boards are slightly thinner and need that support to perform as they should. On the Woodsman range you have a little more in hand, but there is no good reason to go wider, so we would still fit at 300mm and keep the deck feeling the same all over.

Why not 400mm or 600mm?

Those numbers come from timber floor construction, where big structural joists span a room. A deck board is not doing that job. It only has to bridge the gap between one joist and the next, and composite, like any board, will flex if that gap is too big. At 400mm you start to feel a bit of give. At 600mm the deck bounces, the boards can sag over time in warm weather, and you put strain on the fixing clips. It is the single most common reason a finished deck feels cheap, and it is completely avoidable.

When to go tighter than 300mm

There are a few jobs where we drop the centres to around 250mm. Lay your boards on the diagonal and the span between joists gets longer at the angle, so you tighten the frame to keep the same firmness. The same goes for a deck that will take real weight, a hot tub, a heavy planter run, or a commercial space that sees constant footfall. Closer joists cost a bit more in timber, but they are cheap insurance on a deck you want to last.

The rest of the frame matters too

Spacing is only half of it. However you set the centres, every board end has to land on a joist and be fixed down, and where two boards meet end to end you double the joist so both ends are fully supported. Stagger those joints across the deck rather than lining them all up, build a slight fall into the frame so water runs off, and never lay the boards straight onto a solid surface with no gap underneath. Our full composite decking installation guide runs through the do’s and don’ts.

Building the subframe

You can build your frame from treated timber or from our own composite decking joists, which are a good shout for balconies, flat roofs, or anywhere the frame sits close to the ground and damp is a worry. Whatever you use, get it level, get it square, and hold those 300mm centres right across the deck. Do that and the boards will do their part.

If you are still at the planning stage, order a free sample pack and get a feel for the boards before you commit, or browse the full range on our composite decking boards page. Any questions on framing a particular job, give us a call and we will talk it through.